1. Field of the Invention
This invention relates to airbag deployment and inflation technologies, and more particularly to controlling the combustion of an electrically operated propellant to control both the pressure profile and total pressure impulse to inflate the airbag.
2. Description of the Related Art
An airbag is an occupant restraint system consisting of a flexible fabric envelope or cushion designed to inflate rapidly during an automobile collision. Its purpose is to cushion occupants during a crash and provide protection to their bodies when they strike interior objects such as the steering wheel or a window. Modern vehicles may contain multiple airbag modules in various side and frontal locations of the passenger seating positions, and sensors may deploy one or more airbags in an impact zone at variable rates based on the type, angle and severity of impact; the front airbag is designed to only inflate in moderate to severe frontal crashes. Most designs are inflated through pyrotechnic means and can only be operated once.
A central airbag control unit (ACU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors, wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors. The bag itself and its inflation mechanism is concealed within the steering wheel boss (for the driver), or the dashboard (for the front passenger), behind plastic flaps or doors which are designed to “tear open” under the force of the bag inflating. Once the requisite ‘threshold’ has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a fabric bag. As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag's volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant's body, compared to a seat belt alone.
The signals from the various sensors are fed into the ACU, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and “curtain” airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator. The initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas that fills a small chamber with high pressure, high temperature gas in milliseconds that expands (and cools) to inflate the airbag, which is unfolding and drastically increasing the volume at a much lower pressure in approximately 20 to 30 milliseconds. Chamber pressure is typically in the 100's of psi whereas the airbag pressure is in the 10's of psi. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase.
Airbag technologies are being developed to tailor airbag deployment to the severity of the crash, the size and posture of the vehicle occupant, belt usage, and how close that person is to the actual airbag. Many of these systems use multi-stage inflators (e.g. multiple 1-shot inflators) that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the airbag control unit know if someone is occupying a seat adjacent to an airbag, the mass/weight of the person, whether a seat belt or child restraint is being used, and whether the person is forward in the seat and close to the airbag. Based on this information and crash severity information, the airbag is deployed at either at a high force level, a less forceful level, or not at all.
Adaptive airbag systems may utilize multi-stage airbags to adjust the pressure within the airbag. The greater the pressure within the airbag, the more force the airbag will exert on the occupants as they come in contact with it. These adjustments allow the system to deploy the airbag with a moderate force for most collisions; reserving the maximum force airbag only for the severest of collisions. Additional sensors to determine the location, weight or relative size of the occupants may also be used. Information regarding the occupants and the severity of the crash are used by the airbag control unit, to determine whether airbags should be suppressed or deployed, and if so, at various output levels. See Wikipedia “airbag” May 8, 2014.
U.S. Pat. No. 8,317,952 entitled “High Performance Electrically Controlled Solution Solid Propellant” disclosed an electrically controlled propellant comprising a binder, a hydroxylamine nitrate (HAN) based oxidizer and a cross-linking agent. The use of the HAN-based oxidizer overcame the problems with melting of earlier propellant formulations. In one example, this propellant has exhibited the ability to extinguish at ≈350 psi and could be ignited at low voltages. However, subsequent tests indicated that 150 psi was an upper pressure limit for extinguishment. (Col 19, lines 21-24). We are currently applying our controllable solid propellant technology to developing motor and low cost controller technology to a dual-stage tactical rocket motors, in smart automotive airbags, and as an emergency ballast blow/purge system for submarines, replacing the typical all or nothing emergency gas generation ballasts with one that is more controllable. (Col. 20, lines 7-13).